Generally, a crawler type tractor, e.g., a bulldozer or the like vehicle runs such that an output from an engine installed on the vehicle is transmitted to sprockets via a power line comprising a torque converter, a speed changing unit, bevel gears, a steering clutch, a steering brake and a final speed reduction unit and thereby track shoes extending round the sprocket wheels are driven.
On the other hand, a target engine speed of the engine on the crawler type tractor is set to a predetermined value by a throttle lever. Once the target engine speed is set, an output torque generated by the engine is controlled to reach the target engine speed by controlling a quantity of fuel to be injected. Usually, the crawler type tractor is equipped with a deceleration pedal so that the engine output can be reduced to a value corresponding to an extent of depressing of the deceleration pedal which has been depressed by an operator.
Tractive power generated by the engine during running of the crawler type tractor is related to a shoe slip rate representative of shoe slip appearing between the track shoes and the ground surface. As the shoe slip increases, the tractive power increases correspondingly, until the shoe slip rate reaches a predetermined limit value. However, if the shoe slip rate exceeds the predetermined limit value, the track shoes are uselessly caused to slip with the result that an engine output from the engine on the crawler type tractor fails to be effectively utilized as a tractive power. This leads to not only loss of energy but also wear of the track shoes within a short period of time.
Hitherto, e.g., when shoe slip occurs due to increased load during a soil heaping operation with the use of a blade or during a ripping operation with the use of a ripper unit attached to the bulldozer, an operator senses an occurrence of shoe slip and depresses the deceleration pedal to reduce the engine output. At the same time, he actuates a working unit actuating lever to alleviate the engine load to lower the tractive power. In other words, a problem arising due to an occurrence of shoe slip has been heretofore eliminated by the aforementioned complicated actuations which require a highly trained skill.
However, since a controlling operation for adequately compensating the shoe slip by an operator's actuation has been performed by sensing of the shoe slip, depressing of the deceleration pedal with his foot and actuating of levers for working units with his hand, he feels complicated and moreover he is required to pay close attention to his actuations. In practice, since he reduces an output from the engine after an occurrence of shoe slip, it is unavoidable that the shoe slip lasts for a certain period of time. In addition, some operators often cause useless slip due to careless inattention. In this case, it may be impossible to completely prevent an occurrence of such shoe slip. Particularly, when the bulldozer collides against a hard rock-bed during a ripping operation, the result is that the rear part of a vehicle body is raised up without piercing of a ripper into the rock-bed so that the bulldozer is brought in a three-point grounding state wherein the vehicle body comes in contact with the ground surface at three points, two of them being located at the fore parts of both crawler belts and the other one being located at the foremost end of the ripper. Once the vehicle is held in the three-point grounding state, an apparent load is reduced and a vehicle speed increases in excess of a required one. Consequently, the ripper can be pierced into the rock-bed only with much difficulty or it cannot be pierced into the rock bed any more. In such case, it is impossible to release the bulldozer from the three-point grounding state, unless an operator depresses the deceleration pedal to reduce an output from the engine.
In addition, the vehicle has a low shoe slip rate and receives a small magnitude of load at the time of starting movement thereof. Thus, it is natural that a vehicle speed increases in excess of a required one, whereby the ripper is incorrectly pierced into the rock-bed during a ripping operation. Therefore, there arises a problem that a long period of time is required until the ripper is pierced into the rock bed by a predetermined depth. To eliminate this problem, an operator is required to depress the deceleration pedal to reduce a vehicle speed even at the time of starting movement of the vehicle. However, so as to allow the vehicle speed to be adequately raised up via a very low speed, a low speed and an intermediate speed, he should be trained to a considerably high level. This is because if he is an untrained operator, he will not be able to properly deal with the aforementioned malfunction and inconvenience.
In general, with such a crawler type tractor as described above, while a transmission is held in a neutral state, the engine is fully rotated, unless an operator depresses the deceleration pedal. For this reason, to save fuel cost, he is required to depress the deceleration pedal at every time when the vehicle starts its running, while the transmission is held in the neutral state. In a case where he depresses the deceleration pedal after starting movement of the vehicle or at the same time as the starting movement of the vehicle, the engine speed decreases immediately due to an inertia of the engine, resulting in quick starting movement of the vehicle being achieved. Thus, another problem is that correct actuation of the deceleration pedal is absolutely necessary, e.g., to perform a ripping operation at a certain fixed location.
The present invention has been made with the foregoing background in mind.
An object of the present invention is to provide an apparatus for controlling an output from an engine installed on a crawler type tractor wherein an operator is not required to actuate a deceleration pedal during a ripping operation and a three-point grounding state of the vehicle due to incapability of piercing of a ripper into a hard rock-bed.
Another object of the present invention is to provide an apparatus for controlling an output from an engine installed on a crawler type tractor wherein while a transmission is held in a neutral state, an engine speed is automatically reduced to a predetermined one to save fuel cost and prevent quick starting movement of the vehicle.
Another object of the present invention is to provide an apparatus for controlling an output from an engine installed on a crawler type tractor wherein the vehicle can continuously run for a certain operation at an acceptable shoe slip rate without necessity for actuating the deceleration pedal.